1. Field of the Invention
This invention relates to an endoscope system having an image receiving part of many pixels such as a television camera or co-viewing apparatus fitted to an eyepiece part of an endoscope having an image guide with a fiber bundle.
2. Related Art Statement
Recently, an endoscope has recently been used whereby an organ within a body cavity or an interior of a mechanical structure can be observed with an elongated insertable part inserted into the body cavity or, as required, various curing treatments can be made by using treating tools inserted through a treating tool channel.
As in the above mentioned endoscope, there is a fiber scope wherein an object image is transmitted by an image guide having a fiber bundle from an objective lens in a tip part of an insertable part to en eyepiece lens in an eyepiece part and can be observed with a naked eye.
In the above-mentioned fiber scope, television camera connected to the eyepiece part is provided with a solid state imaging device so that a number of persons can simultaneously observe the image. The solid state imaging device is employed as an imaging means or co-viewing apparatus having another image guide.
Now, in the above mentioned fiber scope, as shown, for example, in FIG. 31, an image guide 530 is formed by substantially and closely bundling up (i.e., about several tens of thousands of optical fibers 531 within a diameter of about 10 .mu.m.) for forming respective pixels 541 and therefore producing mesh patterns on the boundaries of the respective fibers 531. On the other hand, such solid state imaging device such as a CCD, BBD or SID is used. As shown, for example, in FIG. 32, in such a solid state imaging device 540, many light receiving regions forming the respective pixels 541 are arranged regularly in the form of a matrix. Therefore, there are problems in that, when an image transmitted by the above mentioned image guide 530 is imaged by the solid state imaging device 540, moire fringes are produced by the arrangement of the fibers and arrangement of the pixels of the solid state imaging device and the picture quality will deteriorates.
In other words, as shown in FIG. 33, as the respective pixels 541 of the solid state imaging device are larger than the respective fibers 531 of the image guide and the spatial sampling frequency by the pixels 541 is smaller than the spatial frequency of the image guide, the nyquist limit as of the imaging system is exceeded and a false signal is generated.
Where a color filter array to obtain a simultaneous type color signal is provided on the front surface of the solid state imaging device, moire fringes will be produced even by the regular arrangement of the respective filters of this color filter array and the arrangement of the fibers.
There is already suggested a means for preventing the generation of such moire fringes wherein such low pass filter as a crystal filter having double refractive actions is arranged between the image guide and solid state imaging device.
An example of an endoscope apparatus provided with such low pass filter is shown in FIG. 34 wherein, within an endoscope insertable part 501, an image guide 530 consisting of a fiber bundle is extended, an objective lens 503 is arranged as opposed to the entrance end at the tip of the image guide 530 and an eyepiece lens 504 is arranged as opposed to the exit end of the image guide 530. Therefore, in the case of an eye observation, the image of an observed object 505 can be observed through this eyepiece lens 504.
Also, in order to pick up an image with a video camera, a television camera unit 507 is removably fitted as an adapter to an observing part 506 containing an eyepiece lens 504 and is provided with a photographing lens 508, a plurality of low pass filters 509 and a solid state imaging device 540 consisting of a CCD so that an output signal of the solid state imaging device 540 may be processed by a camera controlling unit 511 to make a television signal which will be displayed on a monitor 512.
The plurality of low pass filters 509 inserted between the photographing lens 508 and solid state imaging device 540 are formed of such double refractive filters as quartz plates or diffractive lattice filters utilizing a diffractive action.
As shown in FIG. 32, in case the pitch arrangement in the x direction of the light receiving region of the solid state imaging device 540 is 17 .mu.m. and the pitch arrangement in the y direction is 26 .mu.m., in the spatial frequency region shown in FIG. 35, the frequency of moire fringes will be 30 fringes/mm. in the x direction and 20 fringes/mm. in the y direction and no spatial frequency higher than this frequency will be able to be reproduced in the respective directions. Therefore, by passing through quartz plates 509 having such double refractive actions as are shown in FIG. 36 and having a response characteristic showing a trap frequency at 30 fringes/mm. as shown in FIG. 37, the generation of moire fringes in the x direction can be controlled.
However, in the endoscope apparatus of the related art shown in FIG. 34, as moire fringes are not generated in one direction but are generated in a plurality of directions, it is necessary to provide many or usually four or more low pass filters 509 having the optical axis set to be able to erase moire fringes in the respective directions. Therefore, there is a defect in that the television camera unit 507 becomes large in size. More particularly in case this television camera unit 507 is to be provided as an adapter, it must be as small and light as possible. Therefore, there are defects in that the number of the low pass filters which can be contained will be limited and the moire fringes will not be able to be well controlled. Further, there are defects in that the double refractive filters when used as the low pass filters 509 are so expensive that, if many of them are provided, the cost of the entire apparatus will be high.
Also, there are problems in that, if many low pass filters are provided, the contrast is reduced and the resolution deteriorates.
As shown, for example, in the publication of a Japanese patent application laid open No. 71024/1984, removal of moire fringe components with an electric filter from a picture image signal from a solid state imaging device has been suggested. However, there is a problem in that the circuit formation becomes complicated.
Also, in case a co-viewing apparatus is connected to the above mentioned eyepiece part, moire fringes will be produced by the arrangement of the fibers of the image guide within the fiber scope and the arrangement of the fibers of the image guide within the co-viewing apparatus.
As a means of preventing the generation of such moire fringes, it is suggested as shown in the publications of Japanese patent applications laid open Nos. 143125/1980 and 11029/1981 to provide low pass filters between the end surface of an image guide of a fiber scope and the end surface of an image guide of a displaying apparatus. However, there are problems in that, in such a case, the same as in the case of the above described television camera, the structure will be complicated, large and expensive.
By the way, there is known a technique of vibrating the objective side end surface of the image guide of a fiber scope or the objective optical system and the eyepiece side end part of the image guide or the eyepiece optical system in the direction intersecting at right angles with the optical axis in order to improve the resolving power. It is mentioned in the respective publications of Japanese patent applications laid open Nos. 168013/1983, 168015/1983, 53919/1985 and 80605/1987 and a Japanese utility model application laid open No 49208/1982, in U.S. Pat. Nos. 3,016,785 and 4,154,502 and in "Fiber Optics" by New York Academic Press, 1967. However, the prevention of the generation of moire fringes produced by the eyepiece part of a fiber scope and an image receiving means connected to this eyepiece part is not considered in any of these references.